Conventionally, electronic devices, such as CPUs (Central Processing Units), have a specified value for the driving voltage to be supplied during their operation. Therefore, a configuration is used in which an electronic device itself sends a signal indicative of the driving voltage to a power-supply apparatus and the power-supply apparatus supplies electric power at the specified voltage.
In some cases, the driving voltage margin is checked during the development stage or at the shipment stage of an electronic device. If the driving voltage margin is correct, the reliability of the electronic device is increased. This is because, for example, even when, due to a defect with a power-supply apparatus, the value of the actually supplied driving voltage is shifted from the specified voltage value that is requested by the electronic device, the electronic device is still operable.
Checking the driving voltage margin of an electronic device involves preparing a power supply with a voltage at a predetermined value higher than the voltage requested by the electronic device, preparing a power supply with a voltage at a predetermined value lower than the requested voltage value, supplying the driving voltages from these power supplies, and checking whether the electronic device operates properly when the driving voltages are supplied from the power supplies.
An example of a multi-voltage power-supply apparatus will be explained with reference to FIG. 14. FIG. 14 is a diagram of the configuration of a two-voltage power-supply apparatus. The power-supply apparatus illustrated in FIG. 14 includes an interface control unit 61, registers 62_1 to 62_3, an adder 63, and power supplies 64, 65.
The interface control unit 61 writes a value that corresponds to a first voltage to the register 62_1. The interface control unit 61 also writes a value that corresponds to the difference between the first voltage and a second voltage to the register 62_2.
The adder 63 writes the sum of the value written on the register 62_1 and the value written on the register 62_2 to the register 62_3. Therefore, the register 62_3 holds the value that corresponds to the second voltage.
The power supply 64 outputs a voltage that corresponds to the value read from the register 62_1, i.e., outputs the first voltage. The power supply 65 outputs a voltage that corresponds to the value read from the register 62_3, i.e., outputs the second voltage. With the configuration illustrated in FIG. 14, the interface control unit can control different voltage values and maintain the difference between the voltages.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2007-228702    Patent Literature 2: Japanese Laid-open Patent Publication No. 2007-259659    Patent Literature 3: Japanese Laid-open Patent Publication No. 05-276745
Electronic devices have been known that change a request voltage value in accordance with the operational state thereof. The purpose of a dynamic change in the voltage by an electronic device is to save power. For example, a CPU that dynamically changes the voltage decreases the operation clock speed thereof and then lowers the voltage by issuing a voltage change instruction to a power-supply apparatus. The CPU increases the voltage by issuing a voltage change instruction to the power-supply apparatus and then increases the operation clock speed thereof.
The problem with the conventional technology is that it is impossible to check the driving voltage margin of an electronic device that dynamically changes its voltage. Even when two or more voltages are generated with the conventional configuration described above, the value of the voltage used during a check is fixed. When an electronic device changes the request voltage during the check, the supplied voltage is not changed; therefore, the supplied voltage is too high or too low for the changed request voltage even though the margin is included. As described above, with the conventional technology, it is impossible to set a margin voltage value that is appropriate for an electronic device that dynamically changes its operation voltage.